High gloss calcium carbonate coating compositions and coated paper and paper board manufactured from same

ABSTRACT

A paper coating pigment is provided which includes a blend of a first and a second discrete aragonitic precipitated calcium carbonate (PCC) particles. The first particles of the blended pigment have an average particle size (APS) of about 0.4 microns and the second particles of the blended pigment have an average particle size (APS) of about 0.5 microns. The blended pigment preferably includes a weight ratio of about 50:50 to about 80:20 with a 60:40 weight ratio being most preferred, respectively, of the first 0.4 micron particles to the second 0.5 micron particles. Also provided are a method for preparing coated paper, which includes preparing the blended aragonitic PCC pigment, adding a binder, and applying the pigment to the paper basestock in a slurry containing a binder and or additives. Preferably, blended aragonitic precipitated calcium carbonate pigment is present in an amount of from about 30 weight percent to about 85 weight percent of the mixture with about 70 weight percent to about 15 weight percent balance being clay prior to preparing the slurry.

FIELD OF THE INVENTION

The present invention relates to mixtures of calcium carbonate particlesthat are useful in the production of high quality coated papers havinghigh sheet gloss.

BACKGROUND OF THE INVENTION

In paper manufacture, calcium carbonate, because of its excellentwhiteness properties, has been used in coating applications to improvevarious properties such as the brightness of the paper. Both natural andsynthetic calcium carbonates are used in the paper industry. Naturalcalcium carbonate, or limestone, is ground to a small particle sizeprior to its use in paper, while synthetic calcium carbonate ismanufactured by a precipitation reaction and is called precipitatedcalcium carbonate (PCC). Precipitated calcium carbonates are generallypreferred over ground calcium carbonates in paper production in that themorphology, the size, and the size distribution of the particles, aswell as the purity of the as-produced calcium carbonate, can becontrolled.

When used as an additive for the paper industry, precipitated calciumcarbonates are commonly prepared by the carbonation, with carbon dioxidegas, of an aqueous slurry of calcium hydroxide (“milk of lime”). Theprecipitated calcium carbonate pigments are then applied to the paper bycoating the paper with an aqueous slurry containing the precipitatedcalcium carbonate and an adhesive.

Calcium carbonate can be precipitated from an aqueous solution in threedifferent crystal forms: the vaterite form which is thermodynamicallyunstable, the calcite form which is the most stable and the mostabundant in nature, and the aragonite form which is metastable undernormal ambient conditions of temperature and pressure, but converts tocalcite at elevated temperature. The aragonite form has an orthorhombicshape that crystallizes as long, thin needles that may be eitheraggregated or unaggregated. The calcite form exists in several differentshapes of which the most commonly found are the rhombohedral shapehaving crystals that may be either aggregated or unaggregated and thescalenohedral shape having crystals that are generally unaggregated. Allthese forms of calcium carbonate can be prepared by carbonation of milkof lime by suitable variation of the process conditions as is known inthe art.

Although excellent in respect of the whiteness and absorptivity ofprinting inks when used as a pigment for paper coating as compared withkaolin clay consisting of platelet particles, conventional calciumcarbonate pigments suffer a deficiency in that paper coated using thesame is generally poor in sheet gloss. To date, the use of high levelsof precipitated calcium carbonate in coating formulations, particularlysingle-coated applications, has historically resulted in glossdeficiencies compared to kaolin-based formulations. Thus, precipitatedcalcium carbonate particles have been used as pigments in kaolin-basedcompositions in lower amounts, i.e., in amounts of 25 weight percent orless.

RELATED ART

U.S. Pat. No. 5,861,209 teaches aragonitic precipitated calciumcarbonate pigments for coating rotogravure printing papers, a method forthe preparation of the pigment, a paper coated with the coating pigment,and a method for preparing such a paper. The precipitated calciumcarbonate particles have an aspect ratio of from about 3:1 to about15:1, preferably from about 4:1 to about 7:1, and a multimodal particlesize distribution, which is preferably bimodal or trimodal. Preferably,the aragonitic precipitated calcium carbonate is present in an amountfrom about 20 percent to about 100 percent by weight of the coatingpigment. The pigment may also be used with titanium dioxide, talc,calcined clay, satin white, plastic pigments, aluminum trihydrate, mica,or mixtures thereof.

The paper, “In Search of Synergy: Engineering Coatings for MaximumPerformance: Optimizing Pigment Blends for Maximum Performance,” by J.Drechsel (1999 Coating Conference, pp. 413-432), teaches the use of fineparticle size kaolins and fine ground carbonates in coatings to improvethe print gloss of coated papers.

The paper “Structure of the Coating Layer and Optical Properties ofCoated Paper,” by L. Jarnstrom et al., Wochenblatt f. Papierfabrikation17, 736-741, (1996), teaches higher opacity papers and positivesynergistic effects achieved when a precipitated calcium carbonatepigment is mixed with a platelike kaolin for coating compositions.

The paper “Optimized Binder Systems for Natural Calcium CarbonatePigments with Narrow Particle Size Distribution,” by R. Knappich et al.,PTS Coating Symposium (1999), pp. 13E to 13E-16, teaches the use ofnatural ground calcium carbonate pigments with narrow particle sizedistributions as providing a combination of high brightness, highopacity, and excellent coverage for coated paper and board.

The paper, “Factors Governing Print Performance in Offset Printing ofMatt Papers,” by P. G. Drage et al., 1998 TAPPI Coating/PapermakersConference, pp. 413-433, teaches the production of matt pigments formatt and low gloss papers using bimodal blends of coarse and ultrafinecomponents with GCC being focused on as the coarse fraction.

Japanese Pat. App. No. 10232253 teaches a multilayer coated paper forweb rotary offset having dry strength, white paper gloss, multicolorprinting gloss and blistering resistance. The multilayer paper includescoating layers having hollow or hemispheric polymer particles.

Japanese Pat. App. No. 10-340790 teaches a coated paper for offsetprinting having properties of white paper glossiness prepared using anundercoating liquid of a pigment component comprising a wet pulverizedneedlelike or pillar-shaped precipitated calcium carbonate in an amountof 40 weight percent-100 weight percent of the pigment component.

Japanese Pat. App. No. 11-065703 teaches a coated paper for offsetprinting having printing glossiness provided by a coating layer mainlyof pigment containing a 60 weight percent-90 weight percent fusiform wetground causticized precipitated calcium carbonate and a copolymer latexhaving a 50 nm-80 nm average particle diameter and 30 weight percent-50weight percent gel content as the adhesive.

Japanese Pat. App. No. 11-008162 teaches a matt-coated paper for gravureprinting having extremely low white paper glossiness. At least one sideof the base paper is coated with a composition having 75 weightpercent-85 weight percent agglutinative spindle-shaped precipitatedcalcium carbonate having average particle diameters of 3.0 μm-5.0 μm ina secondary particle shape and 15 weight percent-25 weight percent ofkaolin having average particle diameters of 1.0 μm-2.0 μm.

Japanese Pat. App. No. 11-069426 teaches lightweight-coated paper foroffset printing having blank paper glossiness and print glossiness. Thepaper has two coated layers both continuing a pigment and an adhesive,the top coat layer having 50 parts by weight-85 parts by weight ofcalcium carbonate having an average particle diameter not smaller than0.2 μm and smaller than 0.5 μm as the pigment and 8 parts by weight-15parts by weight copolymer latex having 50 nm-70 nm average particlediameter and 50 percent-70 percent gel content as the adhesive bothbased on 100 parts by weight pigment.

Thus, there still remains a need for improved coating grade calciumcarbonate pigments for producing high sheet gloss papers.

SUMMARY OF THE INVENTION

The present invention relates to a paper coating pigment which comprisesa blend of first and second discrete aragonitic precipitated calciumcarbonate (PCC) particles. The first particle of the blended pigment hasan average particle size (APS) of about 0.4 microns and the secondparticle of the blended pigment has an average particle size (APS) ofabout 0.5 microns. The pigment preferably comprises about a 50:50 toabout a 80:20 weight ratio of the first 0.4 micron particle to thesecond 0.5 micron particle, with about a 60:40 weight ratio being mostpreferred.

The present invention also relates to a method for preparing the coatedpaper, which comprises preparing the blended aragonitic precipitatedcalcium carbonate pigment, adding a clay, and applying the pigment tothe paper basestock in a slurry containing a binder and other additives.Preferably, the blended aragonitic precipitated calcium carbonate ispresent in an amount of from about 30 weight percent to about 85 weightpercent of the mixture with the about 70 weight percent to about 15weight percent balance being clay prior to preparing the slurry.

DETAILED DESCRIPTION OF THE INVENTION

Unless otherwise specified, all reference to parts or percent hereinrefer to percent by weight.

The present invention is related to a paper coating compositioncontaining a blend of calcium carbonate particles, a method of using thecomposition to improve sheet gloss, and a process of producing a paperhaving high sheet gloss and the paper made from the process. The calciumcarbonate is preferably a precipitated aragonite (i.e., orthorhombiccrystalline form). When used in pigment formulations, the calciumcarbonate pigment blend of the present invention provides improvement inthe sheet gloss, when compared to typical prior art coating gradecarbonates, and is particularly advantageous in the production ofhigh-gloss papers including paper board.

The improvement in sheet gloss of coated papers using the blendedpigment is unexpected and is attributed to a synergy created by usingparticles having particle size distributions with mean values that arefrom about 0.1 microns to about 0.2 microns in difference in the blendedpigment. In addition, the calcium carbonate pigment of the presentinvention has other clear advantages over other calcium carbonatepigments including its ease of calendering a paper sheet and theresultant sheet gloss and print gloss of coated papers using the pigmentblend.

The calcium carbonate component particles useful in the blended calciumcarbonate pigment blend of the invention are preferably synthesized(i.e., precipitated) by the carbonation with carbon dioxide gas of anaqueous slurry of calcium hydroxide (“milk of lime”) to produce discretearagonitic particles. In a preferred embodiment, preparation of theblended calcium carbonates of the invention is accomplished by mixingcomponent particles having mean particle sizes of 0.40 microns and 0.50microns, which are commercially available from Minerals TechnologiesInc., New York, N.Y., as OPACARB® A40 PCC and OPACARB® A50 PCC,respectively.

More specifically, OPACARB®A40 PCC and OPACARB®A50 PCC are aragoniticprecipitated calcium carbonate particles with average particle sizes ofabout 0.4 microns and 0.5 microns, respectively, having narrow particlesize distributions of ±0.02 microns about the mean.

The average particle size of the particles, according to the presentinvention, were determined by using a Micromeritics Sedigraph 5100Analyzer, an instrument for measuring particle size distributions usingStokes law (see CRC Handbook of Chemistry and Physics, 69^(th) Edition1988-1989, page F-105), which gives the rate of fall of a small spherein a viscous fluid. From this, particle size distributions on a mass(weight) basis and average particle size are determined.

Improved coating results are obtained with the aragonitic PCC pigment ofthe present invention, either alone or in blends with any otherconventional coating pigment. The PCC content of the pigment when mixedwith clay can range from about 30 weight percent to about 85 weightpercent of the coating formulation, with 60 weight percent beingpreferred. The pigment mixture of the present invention is particularlyadvantageous for use in high-gloss printing papers, and may be mixedwith one or more conventional binders, thickeners and/or lubricants asis known in the art. The coating can also contain dilution water in anamount needed to bring the final solids content of the coatings to arange of from about 50 weight percent to about 70 weight percent.

Embodiments of the present invention will now be described by way ofexample only with reference to the following Examples. The followingnon-limiting examples are merely illustrative of the present invention,and are not to be construed as limiting the invention, the scope ofwhich is defined by the appended claims.

In each experimental formulation, 40 total parts of the precipitatedcalcium carbonate pigment was used and mixed with a coating grade clayhaving 72 percent solids using a conventional flat-blade Cowles-typemixer. The clay used for all examples below was ALPHAGLOSS® clayavailable from Huber Corporation. OPACARB®40 PCC; OPACARB®A50 PCC; andALBAGLOS®S PCC particles were provided alone and in combination toevaluate the effect of various calcium carbonate particles.

Specifically, Coating Mixture Nos. 1 and 7 were provided to evaluate theeffect of OPACARB®A40 PCC aragonitic particles and OPACARB®A50 PCCaragonitic particles, respectively, when used alone with ALPHAGLOSS®clay. Coating Mixture Nos. 2, 3, and 4 were provided to evaluate theeffect of OPACARB®A40 PCC aragonitic particles when used in varyingweight ratios ranging from about 40:60 to about 60:40 with ALBAGLOS® SPCC calcite particles traditionally used with clay-based paper coatingcompositions. Coating Mixture Nos. 6 and 5 were provided to evaluateblended aragonitic PCC pigments having OPACARB® A40 PCC aragoniticparticles and OPACARB®A50 PCC aragonitic particles in weight ratiosaccording to the present invention.

The compositions of the coating mixtures prepared are shown in Table 1below with the amounts of the calcium carbonate particles and clay beingpresent following weight percentages. TABLE 1 PIGMENT MIXTURE# 1 2 3 45* 6* 7 ALBAGLOS ® S 25 20 15 PCC (wt. Percent) OPACARB ® A40 40 15 2025 25 20 PCC (wt. Percent) OPACARB ® A50 15 20 40 PCC (wt. Percent)ALPHAGLOSS ® 60 60 60 60 60 60 60 CLAY (wt. Percent)*Calcium carbonate pigment according to the present invention.

After mixing the coating formulations above, binder was added to eachand mixed again using a conventional flat-blade Cowles-type mixer. Eachcalcium carbonate pigment formulation contained the same bindercontaining 11 parts GENFLO® 5905 styrene/butadiene latex available fromGencorp Corporation (now Omnova Corporation), 3 parts hydroxyethylatedstarch available from Penford Starch Corporation as PENFORD® 280 Gum,and 1 part calcium stearate lubricant. A standard paper thickeneravailable from Hercules Corporation as ADMIRAL® 3089 was added to eachcoating sample to achieve a target Brookfield 100 revolutions per minute(rpm) viscosity of 1200 centipoise (cps). Generally, the amount ofthickener used to achieve the target viscosities increased as thecarbonate level increased.

Pigment coatings were formulated at approximately 60 percent solids, andtested for percent solids and water retention character as determined bythe AA-GWR method (Kaltec Scientific, USA). Low shear viscosities incentipoise were measured at 10 rpm, 20 rpm, 50 rpm and 100 rpm using aBrookfield model RVT viscometer. High shear viscosity measurements weremade using a Hercules high shear viscometer from Kaltec Scientific, USA.The Hercules viscosities were run using the following conditions: E bob,400,000 dyne−cm/cm spring constant, 0 rpm-4400 rpm, room temperature.The formulation data for the coatings tested are provided in Table 2.TABLE 2 COATING# 1 2 3 4 5* 6* 7 SOLIDS, percent 60.4 60.2 60.5 60.360.2 60.1 60.2 AA-GWR, grams 89 90 84 91 94 88 97 per square meter (gsm)HERCULES 50.0 47.9 46.5 42.5 45.1 41.0 38.9 VISCOSITY, cps @ 4400 rpmBROOKFIELD VISCOSITY cps @ 100 rpm 1420 1100 1200 1060 1310 1310 1120cps @ 50 rpm 2560 1800 1940 1720 2090 2240 1840 cps @ 20 rpm 5250 36503900 3500 4275 4700 3800 cps @ 10 rpm 9600 6600 7000 6400 7600 8600 7000pH (adjusted to 8.5- 8.5 8.5 8.5 8.5 8.5 8.5 8.5 8.9 with NaOH) AdditionThickener 27 30 30 30 25 30 25*Aragonitic precipitated calcium carbonate pigment according to thepresent invention.

After preparation, the coatings were applied to a 51.1 pounds per ream(70 grams per square meter) paper basestock using a CylindricalLaboratory Coater (CLC-6000). The coat weight target was 9 pounds per3300 square feet. The coated papers were then calendered at 150 degreesFahrenheit on a laboratory supercalender having two sets of rollers withnips providing 800 pounds per square inch (psi).

Standard testing of the coated sheets included paper sheet gloss, printgloss, brightness and opacity. The coated sheet test data are given inTable 3, with the print gloss values being determined using the NancyPlowman Test Method (NPA) and all remaining data being determined usingstandard TAPPI test methods, which test methods will be readilyrecognized by those skilled in the art. TABLE 3 COATING# 1 2 3 4 5* 6* 7SHEET GLOSS, 73.6 72.6 72.1 72.7 75.7 74.6 73.3 percent NPA PRINT 90.692.8 92.1 91.2 91.8 91.6 92.4 GLOSS, percent Rapida SFO Black BRIGHTNESS86.0 85.7 86.0 85.8 86.0 85.9 85.9 TAPPI, percent OPACITY 89.6 90.2 89.789.9 90.1 90.0 90.0 TAPPI, percent COLOR HUNTER L 92.4 92.4 92.4 92.492.5 92.4 92.4 HUNTER A 0.0 0.0 0.0 0.0 −0.1 −0.1 −0.1 HUNTER B 2.3 2.32.3 2.3 2.4 2.4 2.4 Gurley Porosity 83 93 86 91 94 91 95 (Sec @ 10 cc)PPS-10 Roughness 1.32 1.41 1.45 1.36 1.34 1.37 1.42 IGT Pick, 91 91 9176 82 82 76 (cm @ 3.0 m/s) SOLIDS, percent 60.4 60.2 60.5 60.3 60.2 60.160.2*Calcium carbonate pigment according to the present invention.

These data demonstrate the improved sheet gloss that is provided by thecoating pigment of the present invention and also demonstrates that thecalcium carbonate coating pigment can also be used in combination withclay in high quantities without degrading print gloss properties. Whencomparing the sheet gloss of Coating Nos. 6 and 5, a marked improvementis seen when using blended calcium carbonate pigments according to thepresent invention having OPACARB®A40 PCC aragonitic particles andOPACARB®A50 PCC aragonitic particles in weight ratios of from about50:50 to about 80:20, and, preferably, about 60:40.

Moreover, these particle mixtures provide for improved sheet glosswithout the attendant decrease in print gloss normally associated withthe use of higher carbonate contents.

Specifically, the sheet and print gloss ranges of 74.6 percent-75.7percent and 91.6 percent Rapida SFO Black-91.8 percent Rapida SFO Blackachieved using OPACARB®A40 PCC particles and OPACARB®A50 PCC particlesin weight ratios according to the present invention are higher than therespective sheet and print glosses achieved when using OPACARB®A40 PCCalone in Coating No. 1 (73.6 percent, 90.6 percent Rapida SFO Black) andis higher than the sheet gloss and comparable to the print glossachieved when using OPACARB®A50 PCC alone in Coating No. 7 (73.3percent, 92.4 percent Rapida SFO Black). Moreover, a synergistic effectis observed upon using higher ratios of OPACARB®A40 PCC to OPACARB®A50PCC which is contrary to the much lower print gloss (90.6 percent RapidaSFO Black) achieved when using OPACARB®A40 PCC alone with clay (CoatingNo. 1) and the print gloss (92.4 percent Rapida SFO Black) achieved whenusing OPACARB®A50 PCC alone with clay (Coating No. 7).

In comparing the use of the aragonitic blended pigment of the presentinvention with aragonite mixtures containing calcite particles, improvedsheet glosses are also observed. Specifically, when using OPACARB®A40PCC particles and OPACARB®A50 PCC particles in weight ratios accordingto the present invention (Coating Nos. 6 and 5), sheet gloss valuesranging 74.6 percent-75.7 percent were markedly improved over the sheetgloss values of 72.1 percent-72.7 percent obtained using OPACARB®A40 PCCaragonitic particles in weight ratios ranging from 40:60 to 60:40 withALBAGLOS® S PCC calcite particles (Coating Nos. 2-4). Moreover, thismarked improvement in sheet gloss for the blended aragonitic pigment wasobtained with comparable print gloss properties to those containing thearagonite/calcite pigment mix.

It will be observed that the particles utilized in the blends accordingto the present invention exhibit particle size distributions with meanvalues that are from about 0.1 microns to about 0.2 microns indifference. It is envisioned that other blends having similar particlesize distribution differences would exhibit similar synergistic effectson coated sheet properties. Moreover, it is expected that in addition tothe aragonite/aragonite blends, other like-kind mixtures of calciumcarbonate morphologies (eg., calcite/calcite) meeting the above particlesize distribution criteria would exhibit similar synergistic effects oncoated sheet properties.

While embodiments and applications of this invention have been shown anddescribed, it will be appreciated by those skilled in the art thatmodifications and embodiments are possible without departing from theinventive concepts herein described. Therefore, it is intended that theappended claims cover all such modifications and embodiments that fallwithin the true spirit and scope of the present invention. What isclaimed is:

1. A calcium carbonate product for use in coating compositions toprovide a surface finish having high sheet gloss, the productcomprising: first calcium carbonate particles having a first particlesize distribution having a first mean, and second calcium carbonateparticles having a second particle size distribution having a secondmean, the first and second means having a difference of about 0.1microns to about 0.2 microns, the first calcium carbonate particles andthe second calcium carbonate particles being provided respectively in aweight ratio from about 50:50 to about 80:20.
 2. The calcium carbonateproduct according to claim 1, wherein the weight ratio of the firstcalcium carbonate particles to the second calcium carbonate particles isabout 60:40.
 3. The calcium carbonate product according to claim 1,wherein the first calcium carbonate particles and the second calciumcarbonate particles are aragonitic precipitated calcium carbonate (PCC)particles.
 4. The calcium carbonate product according to claim 1,wherein the first calcium carbonate particles have an average particlesize (APS) of about 0.4 microns and the second calcium carbonateparticles have an average particle size (APS) of about 0.5 microns. 5.The calcium carbonate product according to claim 2, wherein the firstcalcium carbonate particles have an average particle size (APS) of about0.4 microns and the second calcium carbonate particles have an averageparticle size (APS) of about 0.5 microns.
 6. The calcium carbonateproduct according to claim 4, wherein the first calcium carbonateparticles have an average particle size (APS) of about 0.4 microns andthe second calcium carbonate particles have an average particle size(APS) of about 0.5 microns.
 7. A paper coating pigment comprising: firstcalcium carbonate particles having a first particle size distributionhaving a first mean, and second calcium carbonate particles having asecond particle size distribution having a second mean, the first andsecond means having a difference of about 0.1 microns to about 0.2microns the first calcium carbonate pigment and the second calciumcarbonate pigment being provided respectively in a weight ratio fromabout 50:50 to about 80:20.
 8. The paper coating pigment according toclaim 7, wherein the weight ratio of the first calcium carbonate pigmentto the second calcium carbonate pigment are provided is about 60:40. 9.The paper coating pigment according to claim 7, wherein the firstcalcium carbonate pigment and the second calcium carbonate pigment arearagonitic precipitated calcium carbonate (PCC) pigments.
 10. The papercoating pigment according to claim 7, wherein the first calciumcarbonate particles have an average particle size (APS) of about 0.4microns and the second calcium carbonate particles have an averageparticle size (APS) of about 0.5 microns.
 11. The paper coating pigmentaccording to claim 8, wherein the first calcium carbonate particles havean average particle size (APS) of about 0.4 microns and the secondcalcium carbonate particles have an average particle size (APS) of about0.5 microns.
 12. The paper coating pigment according to claim 9, whereinthe first calcium carbonate particles have an average particle size(APS) of about 0.4 microns and the second calcium carbonate particleshave an average particle size (APS) of about 0.5 microns.
 13. A processfor producing a paper having high sheet gloss, comprising steps of: a)providing first calcium carbonate particles having a first particle sizedistribution having a first mean, and b) providing second calciumcarbonate particles having a second particle size distribution having asecond mean, the first and second means having a difference of about 0.1microns to about 0.2 microns, c) mixing the first calcium carbonateparticles and the second calcium carbonate particles respectively in aweight ratio from about 50:50 to about 80:20 to form a pigment blend, d)mixing the pigment blend mixture with at least one binder to form acoating slurry, e) coating a paper with the coating slurry to form acoating, and f) drying and calendering the paper to form a coated paperhaving high sheet gloss.
 14. The process according to claim 13, whereinthe weight ratio of the first calcium carbonate particles to the secondcalcium carbonate particles is about 60:40.
 15. The process according toclaim 13, wherein the first calcium carbonate particles and the secondcalcium carbonate particles are aragonitic precipitated calciumcarbonate (PCC) particles.
 16. The process according to claim 13,further comprising steps of providing a clay and mixing the clay withthe coating mixture in step c) in an amount ranging from about 15 weightpercent to about 70 weight percent.
 17. The paper coating pigmentaccording to claim 13, wherein the first calcium carbonate particleshave an average particle size (APS) of about 0.4 microns and the secondcalcium carbonate particles have an average particle size (APS) of about0.5 microns.
 18. The paper coating pigment according to claim 14,wherein the first calcium carbonate particles have an average particlesize (APS) of about 0.4 microns and the second calcium carbonateparticles have an average particle size (APS) of about 0.5 microns. 19.The paper coating pigment according to claim 15, wherein the firstcalcium carbonate particles have an average particle size (APS) of about0.4 microns and the second calcium carbonate particles have an averageparticle size (APS) of about 0.5 microns.
 20. The paper coating pigmentaccording to claim 16, wherein the first calcium carbonate particleshave an average particle size (APS) of about 0.4 microns and the secondcalcium carbonate particles have an average particle size (APS) of about0.5 microns.
 21. The paper product produced-by-the-process according toclaim
 13. 22. A method of improving sheet gloss of a paper productcomprising a step of incorporating composition of claim 1 to a slurryfor coating a base paper.
 23. A method of improving sheet gloss of apaper product comprising a step of incorporating composition of claim 5to a slurry for coating a base paper.
 24. A method of improving sheetgloss of a paper product comprising a step of incorporating compositionof claim 9 to a slurry for coating a base paper.